It is known that the gases in air, such as in particular oxygen and nitrogen, are very important industrially.
At present, one of the non-cryogenic techniques used for producing these gases is the technique referred to as PSA (Pressure Swing Adsorption), which encompasses not only PSA processes proper, but also similar processes, such as VSA (Vacuum Swing Adsorption) or MPSA (Mixed Pressure Swing Adsorption) or TSA (Temperature Swing Adsorption) processes.
According to this PSA technique, when the gas mixture to be separated is, for example, air and the component to be recovered is oxygen, the oxygen is separated from the gas mixture using preferential adsorption of at least nitrogen on a material which preferentially adsorbs at least nitrogen and is subjected to cycles of given pressure in the separation zone.
The oxygen, which is adsorbed little or not at all, is recovered at the outlet of the separation zone; it has a purity, in general, of from 90% to 93%.
More generally, a PSA process for the non-cryogenic separation of a gas mixture comprising a first compound which is adsorbed preferentially on an adsorbent material, and a second compound which is less preferentially adsorbed on the adsorbent material than the first compound, with a view to producing the second compound, cyclically comprises at least:
a step of preferentially adsorbing at least the first compound on the adsorbent material, at an adsorption pressure referred to as the "high pressure", with recovery of at least some of the second compound produced in this way; PA1 a step of desorbing the first compound thus trapped by the adsorbent, at a desorption pressure which is lower than the adsorption pressure and is referred to as the "low pressure"; PA1 a step of recompressing the separation zone comprising the adsorbent, by changing from the low pressure to the high pressure. PA1 they have a shape similar to an ellipsoid of revolution, that is to say b=c; PA1 they are substantially free of sharp edges; PA1 the ratio (a/b) is between 1.08 and 2, preferably between 1.10 and 1.5 approximately; PA1 the contain at least one zeolite or zeolite phase, preferably comprising a type A zeolite or a faujasite, such as X or LSX zeolites. In general, an X zeolite has an Si/Al ratio less than or equal to 1.5. When this Si/Al ratio is equal to approximately 1, such an X zeolite is called an LSX zeolite (for low silica X); PA1 the contain at least one faujasite exchanged with mono-, di- or trivalent alkali or alkaline earth metal cations, transition metals and/or lanthanides preferably cations selected from the group consisting of lithium, manganese, barium, nickel, cobalt, calcium, zinc, copper, magnesium, strontium and iron cations. It is actually usual for metal cations to be incorporated during the synthesis of zeolite particles and/or inserted into the zeolite structure subsequently by an ion-exchange technique; in general, by bringing unexchanged zeolite particles into contact with a solution of one or more metal salts comprising the cation or cations to be incorporated in the zeolite structure, and subsequent recovery of the particles of zeolite exchanged in this way, that is to say zeolite containing a given quantity of metal cations. The proportion of metal cations introduced into the zeolite structure in relation to the total exchange capacity is called the exchange factor, which is expressed in PA1 they contain at least one faujasite, preferably X or LSX, containing at least 70% of lithium and/or calcium cations; PA1 they furthermore contain at least one binder, preferably an inert binder comprising at least one clay, such as kaolin, an attapulgite or a bentonite; PA1 they contain no binder, such zeolites being commonly called "binderless" zeolites; PA1 the X zeolite contains at least 85% of Li.sup.- cations, preferably at least 86% and/or at most 96% of Li.sup.+ cations; PA1 the X zeolite contains at least 80% of calcium cations, preferably at least 85% of calcium cations; PA1 the X zeolite contains at most 15% of Na.sup.+ cations, and/or at most 3% of K.sup.+ cations; PA1 the length (a) of the first axis (A--A) is between 0.4 mm and 3 mm, preferably between 0.6 mm and 2.2 mm, more preferentially between 0.8 mm and 1.6 mm; PA1 the length (b) of the second axis (B--B) is between 0.3 mm and 2.9 mm, preferably between 0.5 mm and 2.1 mm, more preferentially between 0.7 mm and 1.5 mm; PA1 the length (c) of the third axis (C--C) is between 0.3 mm and 2.9 mm, preferably between 0.5 mm and 2.1 mm, more preferentially between 0.7 mm and 1.5 mm. PA1 (a) at least the first gaseous compound is preferentially adsorbed on at least the adsorbent particles according to the invention, and PA1 (b) a gas flow containing predominantly the second gaseous compound is recovered.
Conventionally, the adsorbent particles are placed in one or more adsorbers, so as to form one or more successive layers of adsorbent within each adsorber.
When an adsorber contains several layers or beds of adsorbent particles, these various beds may consist of particles of the same nature, for example beds of zeolite particles, or of different natures, for example a bed of alumina particles followed by one or more beds of zeolite particles.
Furthermore, the adsorbent beds may be placed horizontally within each adsorber, that is to say being stacked on one another, or vertically, that is to say being juxtaposed with one another according to the so-called radial technique.
Moreover, it is known that the separation efficiency for a gas mixture, such as air, depends on a number of parameters, in particular the high pressure, the low pressure, the type of adsorbent material used and its affinity for the compounds to be separated, the composition of the gas mixture to be separated, the adsorption temperature of the mixture to be separated, the size of the adsorbent particles, the composition of these particles and the temperature gradient set up inside the adsorbent bed.
At present, although it has not been possible to determine a general behavior law, knowing that it is very difficult to connect these various parameters with one another, it is also known that the nature and properties of the adsorbent used have an essential role in the overall efficiency of the process.
Currently, zeolites are the adsorbents most widely used in PSA processes, in particular VSA processes.
The zeolite particles used as aggregated adsorbent in PSA processes are synthesized in very fine powder form, which powder is then aggregated and formed into aggregated particles having a size of the order of from 0.4 mm to several millimeters.
The specific purpose of forming the adsorbent particles is to provide a particulate adsorbent whose geometrical characteristics are compatible with the constraints pertaining to, in particular, mechanical strength and adsorption kinetics which are inherent in the separation process employed.
The industrial fabrication of the adsorbent particles is carried out, for example, by aggregating the zeolite powder using a binder, for example a clay, such as kaolin, attapulgite, bentonite or the like.
There are currently two main forms of adsorbent particles, namely on the one hand cylindrically shaped extrudates and, on the other hand, spheres or beads obtained by accretion. In this regard, reference may in particular be made to the documents D. M. Ruthven, Principles of adsorption and adsorption processes, John Wiley & Sons, 1984, p.20, or D. W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, 1974, p.745.
Adsorbents having particular shapes are furthermore also known, for example noncylindrical extruded adsorbents or honeycomb-structure extrudates.
In general, the extruded adsorbent particles known from the prior art are obtained by extruding a paste obtained by mixing zeolite, a binder and water, and optionally additives, such as pore-forming agents or extruding agents, then baking this paste at high temperature, in general at more than 500.degree. C.
However, extruded adsorbents have several drawbacks, namely in particular that their angulated ends promote their attrition when they are being employed, and their very elongate cylindrical geometry facilitates their fracture. Nevertheless, they do in general have a good ratio of their external surface area to their internal volume, which is favorable for the adsorption kinetics.
Spherical adsorbent particles are obtained by accretion from pre-existent particles, called seeds, which are circulated in the presence of binder and zeolite powder while spraying with water, so as to obtain adsorbent spheres by a snowball effect, which adsorbent spheres are then baked at high temperature.
Spherical adsorbent particles of this type generally have good resistance to attrition and crushing, but have a low ratio of their external surface area to their internal volume, which is unfavorable for the adsorption kinetics.
Conventionally, the prior art considers the sphere to be a valid geometrical model for studying diffusion and adsorption phenomena, as mentioned in the document R. T. Yang, Gas Separation by Adsorption Processes, Butterworths, 1987, p.168.
Indeed, it can be stated that almost all studies on the diffusion of gases in aggregated zeolite particles are based on the model with spherical symmetry, see in particular D. M. Ruthven, Principles of adsorption and adsorption processes, p.235-273.